This twelve year old cat comes to you from a rescue shelter with a black deposit in one eye. Is this bad news?
If you read the literature a dark velvety deposit at the edge of the iris like this in a cat spells trouble! One paper from 1998 (Kalishman JB, Chappell R, Flood LA, Dubielzig RR. A matched observational study of survival in cats with enucleation due to diffuse iris melanoma. Veterinary Ophthalmology 1:25-29) compared the survival of cats with iris melanomas in this position with those in the body of the iris and found that once the neoplasm reached the edge of the iris metastasis was common if not inevitable. But I’m not so sure. This melanoma is very dark suggesting that the cells in it are well differentiated melanocytes and thus the neoplasm may be less aggressive than might be thought. I’ve had several cats that have survived well in this state. Should we enucleate? If Kalishman’s paper is right it will already have metastasised and if I am right it never will. Only time will tell who is right!
The other problem is that naevi may develop into melanomas – Kim Young Sam has posted a question about this so here’s a quick reply! We think that naevi may develop into melanomas through Knudson’s two hit hypothesis. Knudson worked on retinoblastoma where some children developed the tumour very rapidly and had it in both eyes while in others it was seen later only in one. He saw that the early onset cases had a germ line gene mutation so that only one mutation in the second chromosome was needde to produce a neoplasm while the allele on both chromosomes needde to be mutated in the later onset cases. Whether this is the case with naevi and melanomas in these cats is unclear but maybe naevi have one allele mutated while frank tumours have both defective. Just a thought! Here’s a link to a good – though to my mind rather complicated – review paper on the mechanisms of development in skin naevi. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3130972/pdf/DRP2011-463184.pdf